A big motivation for it was security posture; it means that Microsoft can now ship security updates to UCRT that everyone can rely on rather than a ton of extra surface area through various multiple versions of runtime libraries.
>>aseipp+(OP)
It had (has) an unsupported, crippled, unversioned msvcrt.dll which if you used it very carefully with a subset of functions, you could write programs which worked fine on Windows NT and up.